Behavioral and physiological effects of an infant-neglect manipulation in a bi-parental, twinning primate: Impact is dependent on familial factors
Introduction
In mammals, infancy is characterized by species-typical interactions with the mother, as well as additional caregivers in some species. These interactions provide specific forms of infant stimulation that ensure survival and promote growth and development under conditions of homeostasis. Therefore, it is widely accepted that whilst complex physiological and neurobiological systems are in a state of marked plasticity in infancy, the necessary dynamics in terms of gene and protein expression on-going in these systems occur against a background of homeostasis. Furthermore, for maintenance of such homeostasis, environmental input in the form of appropriate caregiving is essential. Conversely, this period of dynamic plasticity can also confer increased vulnerability to adverse environmental input, where adversity refers to inputs that lead to stress responses, i.e. loss of homeostasis, with both short- and long-term detrimental effects. By extrapolation, it will be clear that inadequate caregiving, in the form of aggressive over-stimulation or neglectful under-stimulation, either spontaneous or experimental, constitutes the major form of early environmental adversity. There is growing evidence from animal studies that experimental exposure to deprivation of caregiving can induce short-term stress responses in infants (Levine et al., 1992; Sanchez et al., 2001), and can lead to long-term effects on neurobiological, physiological and behavioral phenotypes (Newport et al., 2002; Plotsky et al., 2005; Sanchez et al., 2005) Furthermore, human clinical and epidemiological studies demonstrate that adverse early environments can predispose individuals to develop vulnerability traits for as well as episodes of psychiatric disease, in particular depression and anxiety disorders, across development and into adulthood (De Bellis et al., 1999a, De Bellis et al., 1999b; Heim and Nemeroff, 2001; Pine and Cohen, 2002; Penza et al., 2003; Teicher et al., 2003).
Neglect is the most frequently identified form of child maltreatment. Defining and applying the term neglect is complex in children, as are the actual measurement of neglect and its relationship to specific outcomes. Broadly, neglect can be categorized into the following subtypes: (a) failure to provide food, medical care, clothing, shelter, hygiene and sanitation, (b) lack of adequate supervision, (c) emotional neglect (Dubowitz et al., 2005; English et al., 2005a). Neglect often co-occurs with poverty (Sedlack and Broadhurst, 1996), maternal depression (Kotch et al., 1999), and other types of maltreatment (Dong et al., 2004), each known to impact on child development directly. It has been reported that neglect is associated directly in the child with impaired social and daily-living skills, externalizing and internalizing behavioral problems, and depression and withdrawal (Aber et al., 1989; Dubowitz et al., 2002, Dubowitz et al., 2005; English et al., 2005b). Spontaneous neglect is rare in well-maintained rodent and primate species in the laboratory, but these species do provide important opportunities for detailed description of typical parental care and offspring development, and for controlled prospective studies based on putative neglect-like manipulations.
The homeostasis-stress systems of the hypothalamic–pituitary–adrenal (HPA) axis and sympathetic autonomic nervous system (sympANS) are proposed to exert central roles in the etiology of these inter-relationships. These systems exhibit (i) acute stress responses in infants exposed to caregiving deprivation, leading to short-term changes in neurobiological plasticity with long-term effects on phenotypic development (Hofer, 1994, Hofer, 1994; Blumberg and Sokoloff, 1998; Zhang et al., 2002; Kippin et al., 2004), and/or; (ii) permanently altered basal activity or stressor reactivity, and thereby continuous influence on neurobiological and behavioral phenotypes throughout development and into adulthood (Weaver et al., 2001, Weaver et al., 2004; Ladd et al., 2004; Plotsky et al., 2005). Overall, studies of children experiencing any type of maltreatment provide inconsistent findings in terms of effects on HPA activity (Heim and Nemeroff, 2001; Kaufman and Charney, 2001; Shea et al., 2004). We are not aware of any data on HPA or sympANS activity in neglected children per se. In Romanian orphans that experienced prolonged parental privation (Rutter et al., 2004), basal cortisol levels were high over the daytime hours relative to controls (Gunnar et al., 2001). Maltreated children with affective psychopathology in the form of post-traumatic stress disorder (PTSD) exhibit low peak-basal cortisol levels relative to controls (De Bellis et al., 1999a; Heim et al., 2004). The evidence is quite consistent for the relationship between early life adversity and childhood sympANS activity, in the direction of increased activation (Glaser, 2000). Maltreated children with PTSD exhibit increased systolic blood pressure (BP) and heart rate (HR) (Perry, 1994). Although understanding of long-term mediating mechanisms is sparse, evidence for short- and long-term responses of the HPA and ANS systems to infant–mother manipulations does exist for rats and monkey species, as summarized below.
In rats, a stress hyporesponsive period (SHRP) in terms of synthesis and release of corticotrophin (ACTH) and corticosterone (CORT) has been described; there are reduced basal levels of plasma ACTH and CORT and reduced reactivity to physical challenges, in pups relative to older conspecifics (Walker et al., 1986; Levine et al., 1992). That the SHRP is dependent on an intact pup–dam relationship, and can be disrupted by various pup–dam manipulations, is probably the most striking evidence to-date for the importance of maternal care to at least short-term homeostasis (e.g. Levine et al., 1992; McCormick et al., 1998). For the ANS, rat pups deprived of maternal care for a 24-h period exhibit altered central levels of norepinephrine, reduced HR and increased vasoconstriction (Hofer, 1994, Hofer, 1994). The rodent evidence for robust long-term (adulthood) effects of postnatal social manipulations on HPA function is equivocal. For example, repeated 3-h litter–dam separation was without effect on basal ACTH or CORT but led to an increased restraint stress–response in adulthood in Long–Evans rats (Plotsky and Meaney, 1993; Ladd et al., 2004), whereas in a different strain, Wistar, repeated 4-h pup–dam deprivation did not effect either basal or stress levels in adulthood (Ruedi-Bettschen et al., 2005). Long-term ANS effects of rat postnatal manipulations are poorly studied relative to the HPA system, and this despite the evidence for the marked responses of the sympANS to acute and chronic stressors, as well as the characteristic abnormalities in parameters regulated by this system in affective disorders (Chrousos and Gold, 1992).
Non-human primates do not exhibit a postnatal SHRP, with infancy basal levels of plasma cortisol being at least equivalent to those of adults, and infants exhibiting an immediate cortisol response to removal from the mother and exposure to a physical stressor (Bowman and Wolf, 1965; Pryce et al., 2002; Capitanio et al., 2005). In monkeys, typically species of the Old World macaque genus Macaca, various approaches have been used to model neglect. In the pioneering work of Harlow, the focus was comparative interest in infant–parent attachment and development, based on maternal privation (Harlow, 1958). Maternal privation involves permanent separation of the offspring from the mother within the first few days after birth and hand-rearing in a nursery. This is either without physical contact with any other monkey for the first 6 months (nursery rearing), or in peer groups (peer rearing), with mother-reared offspring providing the comparison group. It does not represent a model of neglect in an extant relationship, therefore. Nursery-reared maternally prived infants exhibit a reduced cortisol stress response relative to mother-reared infants, an effect that emerges within the first month (Shannon et al., 1998). At 3–4 months, nursery-reared maternally prived infants exhibit reduced basal plasma cortisol levels relative to mother-reared infants (Capitanio et al., 2005). At 6 months, peer-reared maternally prived macaques demonstrate a reduced ACTH stress response (Clarke, 1993); for plasma cortisol, peer-rearing led to a reduced stress response in one study (Meyer et al., 1975) and an increased stress response in another study (Higley et al., 1992).
A recent study used repeated separation of rhesus macaque infants aged 3–6 months from the mother, by removal of the mother and leaving the infant in the social group, for 0.5, 3 or 6 h/day and 2–3 days/week (Sanchez et al., 2005). Another approach, termed the variable foraging demand (VFD) paradigm, exposes macaque mothers to an unpredictable food source that elicits reduced maternal care (Rosenblum et al., 1994). In the recent Sanchez et al. study, repeatedly maternally separated females exhibited an increased plasma cortisol response to separation at 5–6 months, and a flattened cortisol basal diurnal rhythm (morning versus evening) at 12 months. Both male and female monkeys exposed to repeated maternal separation exhibited increased whole-body acoustic startle reactivity at 20–24 months (Sanchez et al., 2005). Macaque infants aged 6 months and exposed to maternal separation for the first time at this age exhibit motor hyperactivity and an acute increase in BP, and on the following day, hypoactivity and bradycardia (Reite et al., 1974). In the squirrel monkey, a New World primate, repeated maternal separation at late infancy led to increased glucocorticoid negative feedback in young adulthood (Lyons et al., 2000). In the New World monkey species we report on here, the common marmoset, maternal responsiveness was reduced in a neglect-like manner via tranquilizer administration, and led to offspring exhibiting reduced social play (Chalmers and Locke-Haydon, 1986).
In monkeys, in contrast to rats and mice, there is a postnatal period of several weeks during which there is continuous carrying of the infant by the mother, and in a few species mother and father. Experimental neglect-like intermittent deprivation of care at this stage might therefore constitute an environmental stressor in primates. We have developed such a procedure in the common marmoset (Dettling et al., 2002a). Early deprivation (ED) elicits short-term increases in urinary cortisol, epinephrine and norepinephrine when conducted daily for 0.5, 1, 1.5 or 2 h across weeks 1–4 (weaning weeks 8–10). Basal urinary epinephrine and norepinephrine were increased across months 3–12 (late infancy–adolescence), as was basal systolic BP in months 4–5 (juvenile stage). Behaviorally, ED led to reduced social play in older infants, and to affective and cognitive deficits on conditioned behavioral tasks in juveniles and adolescents (Dettling et al., 2002a; Pryce et al., 2004a, Pryce et al., 2004b). Overall, the ED effects on homeostatic systems were similar to those in children and adolescents exposed to early-life adversity and in depression, and the affective and cognitive behavioral changes were, in direction, analogous to those that occur in depression.
In the present study, our aims were to investigate the effects of ED on social and maintenance behavior, cortisol activity, and cardiophysiologic activity, under basal and psychosocial stress conditions, in juvenile and adolescent common marmosets. With regard to normal development of brain and behavior, as well as in developmental psychopathology, it is now generally accepted that these processes are mediated via environment–gene interactions, with the effect of exposure to a particular environment on the development of complex traits conditional on genetic (polymorphic) factors. It is also clear, from human research, that twin studies have markedly increased our understanding of genetic and environmental contributions to bio-behavioral phenotypes (Moffitt, 2005). The common marmoset is characterized by a reproductive biology of dizygotic twinning and bi-parental caregiving. The developmental effects of ED in common marmosets have been studied using a design in which each breeding pair contributes one pair of ED twins and one pair of control twins, thereby allowing for the identification of main effects of ED and of family (50% shared genotype+100% shared parental environment) and, as an important addition, ED×family interaction effects. As such, we are able to identify family-dependent effects of ED which, although they could not be attributed to specifically genetic or environmental effects, nonetheless are important to a thorough understanding of ED-regulated phenotypes. Here, we were particularly interested in describing the effects, including family-dependent effects, of ED on cortisol activity, given the proposed importance of cortisol at least as a biomarker of and possibly also as an etiological factor in affective disorders.
Section snippets
Subjects and early deprivation (ED)
This study was conducted under experimental permit in accordance with the Animal Protection Act (1978) Switzerland. Nine established breeding pairs of the common marmoset each provided two sets of twin offspring, which were the subjects of the study (Table 1). At any given time during the study, each family group comprised the mother and father and one pair of study twins; existing offspring, including the first set of study twins, were removed or euthanized prior to the birth of study twins.
Urinary AM basal cortisol titers from late infancy to sub-adulthood
Mean AM basal urinary cortisol/CR titers according to early condition, measured from late infancy to sub-adulthood, are provided in Fig. 1A. The observed significant main effect of Age (F(9,90)=50.51, p<0.001) reflected the species-typical monotonic decline in basal cortisol concentrations. There was no significant main effect of early condition (p=0.35); however, there was a significant early condition×parentage×age interaction (F(36,90)=5.40, p<0.001), as well as a significant early
Discussion
Marmoset ED allows for investigation of the short- and long-term effects of daily periods of physical and emotional neglect in a primate species, and at physiological, behavioral and neurobiological levels. Whilst during the first postnatal month ED infants are exposed to isolation that elicits acute physiological responses (Dettling et al., 2002a), they are otherwise reared in social and physical environments very similar to those experienced by their control siblings. With an experimental
Conflict of interest
None declared.
Acknowledgments
We are grateful to J. Kupper and J. Michel for veterinary care and husbandry, and N. Nanz-Bahr, C. Spaete, and R. Morscher for their technical support. This work was funded by the National Science Foundation, Switzerland (Grants 31-55618.98 and 31-67791.02).
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